Definitely, there is a need for replacing prior art container closures manufactured from metal. Metal, for instance, is a heavy material, even in that case where the metal is aluminum. Frequently, a metal cover results in sharp edges when the cover is torn up, which of course is a major drawback from a damage point of view, not only when opening but also when handling the cover and especially when leaving a can and the cover attached thereto out in the countryside.
In a number of countries prohibitions have already been launched against spreading around loose, sharp metal parts, for instance metal pieces torn away from cover panels.
Although the problems relating to such metal pieces have been solved by means of a tongue or a flap which is still attached to the container after the tearing operation, the drawback remains that metal tongues which are torn away from a closure panel necessarily have to be pushed down into the package before pouring the filling goods. This results in an undesired contaminating effect on the contents of the package.
The prior art container closures of metal necessitate a large input of production machinery and the manufacturing requires large energy quantities. Because it all goes around extremely complex machinery, and because ultrahigh production speeds are used for making the production competitive, it is realized that the pattern of movement in the machinery has to be the simplest possible in order not to waste time.
Thus, the traditional application of a pull ring on the panel requires that the ring be placed centrally. If not, time will be wasted for orientation of individual packages, which is not possible in practice.
The technique used for metal closures generally involves punching, deep-drawing, riveting and so called curling and birdling, respectively, for mounting a cover on to the end of a container casing. Before this is possible, however, at least two of said manufacturing steps have to be completed, generally in combination with a final adjustment step.
In addition to the cost of material, the manufacturing cost will then be an economically heavy burden for manufacturing said closure end pieces, which as such actually do not have any intrinsic value.